Method and apparatus for demagnetization of multipole magnetic devices



Dec. 24, 1968 D. J. RENNER 3,418,542

METHOD AND APPARATUS FOR DEMAGNETIZATION OF MULTIPOLE MAGNETIC DEVICES Filed April 18, 1966 5 Sheets-Sheet 2 INVENTOR. DONALD J. RENNER BY MAHONE BY Y, MILLER 8 RAMBO M K M ATTORNEYS D. J. RENNER 3,413,542 METHOD AND APPARATUS FOR DEMAGNETIZATION OF MULTIPQLE MAGNETIC DEVICES 3 Sheets-Sheet 5 Dec. 24, 1968 Filed April 18, 1966 0 M E M R M T W Mm W m M F M mm M w mm M W M UD m m E E IJ P. a L. D D NZ L m M U m 2 I m m 0 .W MU [.5 R 5 W D ,H P h L YA A I". n W M [Q n Z M N I. m+ n I14 L. I. Ina L Rm E n Um m M u H. M ||2T 1h 6 ...R I 5 L I 4 1\3 v s 3 d 2 N 8. i623 VS E 2529: d I I wfimfimm L a: 623 X3. 2529:

MAGNETIC FIELD INTENSITY (H) M ATTORNEYS United States Patent O 3,418,542 METHOD AND APPARATUS FOR DEMAGNETIZA- TION F MULTIPOLE MAGNETIC DEVICES Donald J. Renner, Columbus, Ohio, assiguor to F. W. Bell, Inc., Columbus, Ohio, a corporation of Ohio Filed Apr. 18, 1966, Ser. No. 543,212 9 Claims. (Cl. 317-1575) This invention relates, in general, to demagnetization of magnetized materials or devices such as permanent magnets. It relates, more specifically, to a method and apparatus for effecting the simultaneous, uniform demagnetization of a device having a multiplicity of poles.

In accordance with prior art methods and apparatus for producing magnetized devices or magnets, the devices or magnets are first magnetized to the saturation level of the particular material and then subjected to a demagnetization process in which the residual magnetic flux density is reduced to a predetermined, desired level. This is a conventional procedure for not only producing magnetic devices with a desired strength but for improving the stability of the magnetic device. Demagnetization of a conventional dipole magnetic device is effectively accomplished by positioning the device adjacent a magnetic field-producing, inductive device which is selectively energized to produce a relatively negative magnetic field intensity which opposes and reduces the residual magnetic flux density. The magnitude of the negative magnetic field intensity applied to the device determines the degree of reduction of the magnetic flux density and the magnetic field intensity for demagnetizing is sequentially increased in incremental steps to decrease the magnetic flux density of the device to the desired level.

Prior art apparatus and methods for demagnetization have only been found eifective for demagnetization of the well known dipolar magnetic devices, such as the ordinary permanent magnet, and have not been found capable of effectively or efiiciently demagnetizing devices having a multiplicity of poles, such as four or more. In such multi-pole magnetic devices, the demagnetization of any one specific pole will have an effect on the magnetic flux density of adjacent poles and localized variations in the magnetic characteristics of a multipole magnetic device at the several poles make it necessary to use correspondingly different magnetic field intensities at each of the poles in an attempt to obtain uniformity as to the resultant magnetic flux density of the several poles. Consequently, the application of prior art dipolar demagnetization apparatus and methods to the demagnetization of such multi-pole magnetic devices has required the utilization of trial and error techniques in attempting to reduce the magnetic flux density of each of several poles to a substantially identical level. Such trial and error techniques that have been utilized in the application of conventional, dipolar demagnetizing methods and apparatus to multi-pole magnetic devices often result in the production of multi-pole magnetic devices having substantial nonuniformity of flux density as between the several poles. These prior art techniques also require a relatively large amount of time to provide the necessary degree of demagnetization and must be manually controlled and closely monitored. Even with such close moni toring and control, a relatively large percentage of a production run may be found to be unsatisfactory in that the level of magnetic flux density of each of several poles may differ from the desired value or relative to each other as to present a variation which exceeds the maximum acceptable deviation from a desired value for the particular magnetic devices.

It is, therefore, the primary object of this invention to provide an apparatus and method for demagnetization of multi-pole magnetic devices.

It is a further object of this invention to provide a demagnetization apparatus and method for demagnetizing multi-pole magnetic devices which effects the simultaneous demagnetization of all poles to a predetermined, magnetic flux density within the limits of an accepted tolerance or predetermined deviation.

It is another object of this invention to provide demagnetization apparatus and method for demagnetizing multi-pole magnetic devices through automatically controlled apparatus and capable of effecting the simultaneous demagnetization of all poles to a predetermined mag netic flux density.

It is also an object of this invention to provide a method of demagnetization for multi-pole magnetic devices in which the demagnetization of all poles is etfected simultaneously in two steps with the magnetic flux density of each pole being independently reduced to a predetermined value slightly greater than the ultimate desired value in a first step and simultaneously reducing the magnetic flux density to the ultimate desired value in a second step.

These and other objects and advantages of this invention will be readily apparent from the following detailed description of an embodiment thereof and the accompanying drawings.

In the drawings.

FIGURE 1 is a schematic diagram of the electrical circuitry of a demagnetizing apparatus embodying this invention for demagnetizing a four-pole magnetic device.

FIGURE 2 is a top plan view of a magnetic device holder showing the relative location of the demagnetizing magnetic field devices and the magnetic field detectors responsive to the magnetic field of the device.

FIGURE 3 is an elevational view of the magnetic device holder of FIGURE 2 partially sectioned to further illustrate the relative location of the demagnetizing field producing device, the magnetic field detectors and the magnetic device.

FIGURE 4 is a graphic representation of the demagnetization process of the four-pole magnetic device as demagnetized by the apparatus illustrated in FIGURES 1, 2 and 3.

FIGURE 5 is a graphic representation of the demagnetization characteristics of the several poles of the multipole magnetic device.

Having reference to FIGURES 2 and 3 of the drawings, a magnetic device M with which the apparatus of this invention may be utilized in the application of the method of demagnetization is seen to comprise a circular ring or annularly-shaped disc having a multiplicity of magnetic poles. This particular magnetic device M is formed with four magnetic poles and is illustrated for example only and is not to be considered as a limitation on the application of the apparatus and method of this invention. The poles are angularly displaced relative to each other at about with each of the poles being effectively concentrated for illustrative purposes at the indicated north (N) and south (S) positions of FIGURE 2. The north and south pole indications of FIGURE 2 relates to the demagnetizing, magnetic field-producing devices of the apparatus and are disposed to coincide with the effective concentrated magnetic poles of the device to be subjected to a demagnetizing operation. In FIG- URE 2, the broken arcuate lines are representative of the magnetic field produced by the magnetic device M and are not representative of the magnetic field produced by each of the demagnetizing devices 10.

A holder 11 for supporting the demagnetizing devices 10 in the desired relationship for the particular magnetic device M comprises a cylindrical body 12 having a relatively smaller diameter, coaxially disposed cylinder 13 formed on one end face thus providing an annular surface for the support and positioning of the magnetic device M. The smaller diameter cylinder 13' is adapted to project through the center opening of the annularly shaped magnetic device M and forms an orientating means for properly positioning the magnetic device M on the holder 11. The magnetic device M may be provided with an indexing notch N for facilitating the location of the poles formed on the device. For cooperation with the indexing notch N, a mating indexing projection 14 may also be formed on the cylindrical surface of the smaller diameter cylinder 13. This projection 14 will assist in locating the magnetic device M in the desired predetermined relationship to the holder and thus locate the several magnetic poles of the device M in predetermined relationship to the demagnetizing devices carried by the holder 11. The illustrated holder 11 is preferably formed from a nonmagnetic material to avoid interference with the magnetic field produced by the magnetic device M.

Each of the demagnetizing magnetic field-producing devices 10 comprises a suitably formed coil which is adapted to be positioned in a socket 15 formed in the cylindrical body 12. Each device 10 is disposed in a similar socket 15 at the desired. angular displacement with the axis of the magnetic field produced by the devices 10' being substantially normal to the surface of the cylindrical body 12 on which the magnetic device M is positioned. This arrangement will thus provide a magnetic field which is most effective in demagnetizing the magnetic device M. The condoctors 16 connecting with the coil or demagnetizing device 10 extend through an aperture 17 opening at the opposite face of the cylindrical body 12 for convenience in connection with the apparatus of this invention. If desired, the socket 15 may be filled with a suitable insulating compound .to secure the demagnetizing device 10 in the socket and provide the necessary insulation.

During the course of the demagnetizing operation, it is necessary that the magnitude of the magnetic flux density of the device M be ascertained to assure control of the demagnetization operation in accordance with the method of this invention. A determination of the magnetic flux density may be readily accomplished by means of a magnetic field-responsive device, such as a Hall-effect device 18. Each Hall-effect device 18 is mounted in a suitable protecting structure and is positioned in the holder 11 adjacent a respective one of the demagnetizing devices 10 and the associated pole of the magnetic device M. As best shown in FIGURE 3, each Hall-effect device is disposed in a cylindrical bore 19 extending axially through the cylindrical body 12 and smaller cylinder 13 of the holder. The several conductors 20 connected with the Hall-effect device extend outwardly from the cylindrical body 12 for connection with the apparatus. The bore 19 may also be filled with a suitable electrical insulating compound to securely position the Hall-effect devices 18 in the desired position and provide the necessary protection for the conductors 20. All of the Hall-effect devices 18 are placed equidistantly from the center of the cylinder 13 and, therefore, are spaced equidistantly from the effective concentrated pole of the magnetic device M. Thus, the Halleft'ect devices 18 will be primarily responsive to the magnetic field emanating from the respective pole and, in a uniform magnetic field condition wherein all poles have a magnetic flux density of equal magnitude, will provide an indication which is substantially identical.

Having reference, specifically, to FIGURE 1, the demagnetizing magnetic field-producing devices or coils 10 are shown connected in circuit with a suitable power source which is capable of providing the necessary energy. This power source may include a series regulator 21 which is connected to a source of direct current power and is operable to control the demagnetization. Connected in shunt relationship with each of the coils 10 is a suitable capacitor 22 capable of being charged and of storing sufiicient electrical energy to energize the coils 10 upon discharge of the capacitor through the coil for effecting a demagnetization of a magnetic device. A series charging resistor 23 is connected between the capacitor 22 and the series regulator 21 to limit the charging current to an acceptable value. Connected in series with each of the coils 10 is a respective switching device 24 which is operable to control the flow of current through the coil 10 and the formation of the respective demagnetizing magnetic field. In the illustrated embodiment, this switching device 24 may comprise a silicon controlled rectifier which may be triggered from a non-conducting state to a conducting state by the application of a gating voltage to the gating terminal 24a.

The operation of this basic demagnetizing circuit consists of the alternate charging of the capacitor 22 and subsequent discharging of the capacitor through the coil 10 to produce the demagnetizing magnetic field. The resultant magnetic field intensities may be varied as desired through appropriate adjustment of the series regulator 21 to charge the capacitor 22. to a predetermined voltage. During the capacitor-charging portion of a cycle, the switching device 24 will be in a non-current conducting state and no current will flow through the coil 10. After the capacitor 22 has been charged to a predetermined voltage, a trigger voltage is applied to the gating terminal 24a of switching device 24 resulting in current conduction through the coil 10 and discharge of the capacitor 22. Current conduction through the coil 10 will continue until the capacitor 22 has been discharged and the voltage reduced to the cutoff point of the switching device 24. When the voltage has been reduced to the cut-off point or substantially zero, the switching device 24 will return to a nonconducting state and prevent further current conduction through the coil 10. At this time, the capacitor 22 may again be charged for a subsequent discharge cycle through the coil 10. As in accordance with conventional demagnetizing operations, the electrical energy stored by the capacitor, as determined by the voltage, is sequentially increased in predetermined increments for each subsequent and succeeding cycle for increased energy transfer to the magnetic field producing devices for a stepwise increase in the demagnetization. This process is continued until attainment of the desired magnetic flux density in the magnetic device.

The series regulator 21 is a device employing well known circuitry which provides an output voltage which is dependent on an input signal. The input signal to the series regualtor 21 will thus limit the output voltage of the series regulator to a proportional value of the voltag of the DC. power source. Through appropriate adjustment of the input signal to the series regulator, the output voltage may be sequentially increased in predetermined increments to provide stepwise increases in the magnetic field intensity for demagnetization of the magnetic device M. The control of this input signal for the series regulator 21 may be provided by manually operated apparatus and circuitry or, as in the case of the present invention illustrated herein, through circuitry providing automatic sequential control. Each channel of the apparatus associated with a demagnetizing device 10 may thus be independently controlled to perform a demagnetizing operation with respect to a specific pole of the magnetic device M.

In accordance with prior art techniques and methods, the demagnetizing operation is initiated after the magnetic device has been first magnetized to saturation and is continued until the desired magnetic flux density is obtained. This technique is satisfactory in the instances where a single dipole type magnetic device is being processed; however, where a multi-pole magnetic device must be processed in this manner, the inter-relationship and interaction of each of the poles of the device and the localized variations in magnetic properties of the material will prevent such a simple process from producing a multi-p0le magnetic device having a satisfactory field pattern with each pole having the desired magnetic flux density. Accordingly, it is necessary to continuously monitor the magnetic flux density of each of the poles and independently control the demagnetization of the respective poles to obtain the optimum uniform magnetic flux density. Such manual monitoring and control of the demagnetization process substantially increases the cost of production and thus the production of multi-pole magnetic devices in accordance with prior art procedures has utilized only the minimum control necessary to approximately obtain an ultimate, desired magnetic flux density for the several poles which may deviate from the ultimate, desired flux density to a relatively large extent. Demagnetization in accordance with prior art practices is also incapable of consistently producing multi-pole magnets wherein the interpole deviation is within prescribed limits. Consequently, the rejection rate of such magnetic devices demagnetized in accordance with prior art practice on production runs has been relatively high which further increases the per unit cost of production.

One of the characteristics of a magnetic device which has prevented the application of prior art, simple dipolar demagnetization apparatus and methods to multi-pole demagnetization is the variation of demagnetization characteristics occurring in localized areas of a magnetic material. In multi-pole magnetic devices, the demagnetization characteristics of the material forming each of the poles of a particular magnetic device may be so substantially different as to prevent the utilization of a common control in performing the demagnetization operation. Each of the poles may be demagnetized at a relatively different rate for a particular demagnetizing magnetic field intensity. This effect is diagrammatically illustrated in FIG- URE 5 wherein it can be seen that subjection of each pole of a four-pole magnetic device having the four respective demagnetization characteristics to a specific magnetic field intensity will result in a substantial deviation in the relative magnetic flux density for each of the poles. In addition to dissimilarities in the demagnetization characteristics, the saturation level of magnetization for each of the poles may be dissimilar. This is also illustrated in FIGURE 5 wherein each of the poles is seen to have a different magnetic flux density for the application of a zero magnetic field intensity to the magnetic device. Thus, it will be readily seen that the application of a specific demagnetizing or relatively negative magnetic field intensity H to each of the poles will produce the respective resultant magnetic flux densities B through B It will be noted that the magnetic flux densities of the respective poles increase slightly from the value attained b the application of the magnetic field intensity H to the in dicated densities B through B; after removal of the field as a consequence of the minor hysteresis effect. These magnetic flux densities of the respective poles are widely varied and may further exceed the differences noted at the saturation level for a zero magnetic field intensity. An increase or decrease of the applied magnetic field intensity from the illustrated magnetic field intensity H will also result in differences in the relative magnetic flux density of the respective magnetic poles.

The nonlinearity of the demagnetization characteristics of each of the respective poles of a multi-pole magnetic device prevents the application of a single step type method and a simple multi-pole or multi-channel demagnetization apparatus with only the most simple control system for continuous demagnetization from the saturation level to the desired magnetic flux density. It will be obvious from FIGURE 5 that simultaneous demagnetization -by subjection of all poles to the same magnetic field intensity will result in dissimilar magnetic flux densities for the respective poles as determined by the specific demagnetization characteristics of the material forming each pole of a particular magnetic device.

The effect of such dissimilar demagnetization characteristics on the simultaneous demagnetization of a multipole magnetic device is obviated by the apparatus and the method of this invention for performing the demagnetization operation. This method is graphically illustrated in FIGURE 4 and, in general, consists of performing the demagnetization operation in two steps by subjecting each pole to a pulse-form magnetic field intensity with successive pulses being incrementally increased in magnitude. The pulse-form current 1 graphically illustrated in FIGURE 4 is the current through the demagnetization coil 10 and is, therefore, representative of the applied magnetic field intensity. Demagnetization of the magnetic poles is most effectively accomplished through increasing the magnitude of the current pulses through the demagnetization coil 10 and the resultant demagnetizing magnetic field intensity in successive, relatively small incremental steps over a predetermined time interval. Thus, the demagnetization will appear as a steptype Wave form on a time basis. In the first step, each of the poles is demagnetized to a nominal predetermined magnetic flux density which is only slightly greater than the ultimate desired flux density. The control of the demagnetization of each pole is independent of each other pole and is continued only until the magnetic flux density of the respective pole has been reduced to the nominal predetermined value or prelevel off-flux density. As each pole attains the prelevel-off magnetic flux density, further decreases in the magnetic flux density are delayed until 'all of the poles have reached this nominal prelevel-off flux density. As the magnetic flux density of a pole reaches the prelevel-off magnitude, the magnetic field intensity is continued to be applied but is not further increased until all poles have attained the prelevel-off magnetic flux density. Only the demagnetization current for pole number 3 is illustrated in FIGURE 4 and illustrates the discontinuance of further demagnetization when the magnetic flux density of this pole reaches the nominal prelevel-off magnitude. During this interval, the magnitude of demagnetizing current pulses will be maintained at the last attained level until initiation of the second step of the demagnetization operation. The poles first to reach this prelevel-off flux density will remain at substantially this value until all of the poles have been demagnetized to the prelevel-off magnetic flux density. Continued application of demagnetizing pulses of the same magnetic field intensity will substantially prevent the magnetic flux density of an adjacent pole from affecting the flux density of a pole which first reaches a predetermined value.

The second step of the demagnetization operation is initiated when all of the poles have been demagnetized to a magnetic flux density equal to or less than the nominal prelevel-off value. During the second stepeach of the poles is again subjected to pulse-form magnetic field intensities which are successively increased in predetermined increments. The demagnetization process continues With respect to each pole until that pole has been demagnetized to the desired magnetic flux density. All of the poles will reach the desired flux density at approximately the same time regardless of dissimilarities in the respective demagnetization characteristics as the second step of the demagnetization results in operation on only a relatively small portion of the demagnetization characteristic. By appropriately selecting the nominal prelevel-off magnetic flux density to be only slightly greater than the ultimate desired magnetic flux density, the effect of differences in the demagnetization characteristics of the materials forming each of the specific poles may be minimized as a result of minimizing the total change in magnetic field intensity required toeffect the second step of the demagnetization operation.

As a practical matter, achievement of the ultimate desired magnetic flux density is difficult, if not impossible, and flux densities which deviate slightly from the desired are determined to be acceptable. Acceptable deviations, either above or below the desired, are defined by appropriate production standards or specific criteria for the particular magnetic device and are indicated in FIG- UR-E 4 as maximum and minimum limits. The pulseform magnetic field intensity is preferably increased by increments which are effective in decreasing the flux 7. density of each of the poles, at each pulse, by an amount which is slightly less than the difference between the maximum and minimum allowable flux densities. Thus, if a particular magnetic field intensity applied to a pole results in a decrease of the magnetic flux density to a magnitude slightly above the maximum allowable, a succeeding pulse having an incrementally greater value is applied and thereby decreases the magnetic flux density to a magnitude within the prescribed maximum and minimum limits. Irrespective of the magnitude of the magnetic flux density at the point immediately preceding the last pulse of applied magnetic field intensity, the applied magnetic field intensity for the last pulse will be effective in decreasing the flux density to a value between the maximum and minimum allowable limits but will not decrease the flux density to a value below the minimum allowable flux density. 1

Greater precision in demagnetization may be readily achieved by the method of this invention through decreasing the-difference between the nominal prelevel-off flux density and the ultimate desired flux density and decreasing the incremental increase in magnetic field intensity. A decrease in the incremental increase in mag netic field intensity will permit reduction of the allowable deviation as determined by the maximum and minimum limits provided that the relative relationship of the nominal prelevel-off magnetic flux density to the desired flux density is also decreased proportionally.

The multi-pole demagnetization apparatus for effecting the demagnetization of a multi-pole magnetic device in accordance with the method of this invention is illusrated in FIGURE 1. The illustrated apparatus includes four separate channels for independent control and operation of the demagnetizing magnetic field-producing devices for each of the poles. All of the channels are of identical construction and the following description as it relates to a single channel is equally applicable to the other channels. This apparatus also continuously monitors the magnetic flux density of each of the poles of a magnetic device undergoing demagnetization and provides an appropriate, related control signal for automatic operation of the demagnetization process. This process is automatically performed as briefly outlined in the -'preceding paragraphs.

A signal for control of the series regulator 21 is provided by a digital-to-analog converter 25 responsive to a timing pulse to provide a specific control signal to the series regulator at a predetermined time resulting in charging of the regulator at a predetermined time resulting in charging of the capacitor 22. In addition to the provision of the control signal at a predetermined time, the signal from the converter 25 is effective to control the output of the series regulator 21 by increasing the output thereof in uniform increments for each timing pulse received by the converter. The sequential, incremental increases in the output voltage of the series regulator 21 produce step increases in the current I through the respective demagnetizing device 10 and a consequent increase in the pulse-form magnetic field intensity applied to the magnetic device.

The timed operation of the digital/analog converter 25 is controlled by a pulse generator 26 which provides a continuous series of timing pulses. The timing pulses from the pulse generator 26 control the converter 25 through a logic circuit 27 of the AND type having a first input connected with the pulse generator 26.

A second input to each AND circuit 27 is provided by a level sensing switch 28 connected to a gaussmeter 29 which has a respective one of the Hall-effect devices 18 forming the probe thereof. The level sensing switch 28 is operative to provide an output to one of the inputs of the AND circuit 27 whenever the level of magnetic flux density of the magnetic device as determined by the gaussmeter is above a predetermined value. Since there are two steps in the operation of this apparatus in accordance with the method of this invention to effect the demagnetization, the level sensing switch is designed to provide an output to the AND circuit 27 at either of the two states; that is, the switch 28 will provide an output when the magnetic flux density of the specific pole is' greater than the nominal prelevel-oif magnetic flux density in its first state of operation and will provide an output when the magnetic flux density is greater than the ultimate desired magnetic flux density. A change in the state of operation of the level sensing switch 28 is effected by a reference shifter circuit 3i). The operation is such that the switch 28 will provide an output during the time that the magnetic flux density as determined by the gaussmeter 29 is above the nominal prelevel-otf magnetic flux density and will cease to provide the output once this magnitude of flux density is reached. No further output will be provided by the switch 28 until its refer-' ence is shifted by operation of the reference shifter 30 to the second state of operation wherein an output will again be provided for as long as the magnetic flux density of the specific pole of the magnetic device, as determined by the gaussmeter 29, will be above the ultimate desired magnetic flux density.

A timing pulse from the pulse generator 26 and an output signal from the level sensing switch 28 must be concurrently applied to the AND logic circuit 27 to provide a timing pulse to the digital/analog converter 25. Upon receipt of each succeeding pulse, the control signal from the digital/analog converter 25 will be sequentially stepped to a higher value in successive increments and transmitted to the series regulator 21 to sequentially increase the output thereof.

As previously described, the output of the series regulator 21 is utilized in charging the capacitor 22 to a value as determined by the operation of the digital/ analog converter 25 before subsequent discharge through the coil 10 to produce the demagnetization field. Discharge of the capacitor 22 is controlled by the SCR switching device 24 which may be triggered to a current conducting state by the application of a suitable trigger signal to the gating terminal 24a. This trigger signal is also provided by the pulse generator 26 which is connected to the gating terminal 24a through an AND'logic circuit 31 and an OR logic circuit 32. A demagnetization cycle is terminated by sufiiciently discharging the capacitor 22 whereby a sustaining current is no longer supplied the SCR switching device 24 and the SCR will return to a nonconducting state. In order to obtain the necessary discharge of the capacitor 22, it is necessary to interrupt the charging operation of the series regulator for the discharge time interval. Interruption is readily obtained through inclusion of a gating circuit in the series regulator 21 which operates to open the output circuit of the regulator. This gating circuit may be of the type which is normally closed but becomes open circuited for a'predetermined time interval on receipt of a suitable signal. In this circuit, this signal is the trigger signal applied to the gating terminal 24a and is applied to the gating circuit by interconnection of the series regulator 21 with the output of the OR logic circuit 32.

When the magnetic flux density of the magnetic device reaches the prelevel-otf magnitude, the level sensing switch 28 will be operative to prevent transmission of a gating signal to the AND circuit 27 thereby suspending further operation of the digital/ analog converter 25 as to continued, sequential increasing demagnetization. It is desirable that the poles of the magnetic device which first attain the nominal prelevel-oif magnetic flux density be continued to be pulsed with a demagnetizing magnetic field intensity of the last attained'value to further assure a substantially uniform magnetic flux density for all of the poles at the initiation of the second step of the demagnetizing operation. The series regulator 21 and the value and the coil will thus be enabled to continue producing the demagnetizing field on discharge of the capacitor. Triggering pulses will continue to be applied to the gating terminals for cyclic energization of the respective coils 10.

An output signal from each level sensing switch 28 is also applied to the reference shifter 30. The reference shifter 30, which may also be a logic circuit, is responsive to the simultaneous application of a signal from each of the channels to provide an output signal. Each level sensing switch 28 is adapted to provide a signal for operation of the reference shifter when the magnetic flux density of the respective pole reaches the nominal prelevel-off magnetic flux density. When such a signal is' received simultaneously from each of the level sensing switches 28, a reference shifting signal will be provided by the reference shifter 30 for triggering the respective level sensing switches 28 to the second state of operation. In this second state of operation, the level sensing switches 28 will again provide a signal to the AND circuit 27 resulting in continued operation of the digital/analog converter to permit further increase in the magnitude of the demagnetization pulses applied to the respective magnetic poles. This second state of operation will then continue until the magnetic flux density of the respective pole attains the ultimate desired magnetic flux density. Attainment of the ultimate desired magnetic flux density will again result in elimination of the output signal from each of the level sensing switches 28 to the respective AND circuit 27 and prevent the transmission of further timing pulses to the digital/analog converter 25 which would further increase the magnitude of the demagnetization field.

A demagnetization operation for a particular magnetic device M is concluded when all level sensing switches 28 reach the second level of operation. At this time, the demagnetization operation is terminated and the circuit may be returned to its initial state through appropriate resetting of the several circuit elements and removal of the magnetic device M from the holder 11. As each level sensing switch 28 reaches the second level of operation, a signal is transmitted to the reference shifter 30 to terminate the demagnetization operation and initiate resetting of the several circuit elements to their initial state. Each of the digital/ analog converters 25 must be reset to a predetermined starting point for a subsequent demagnetization operation. Also, the reference shifter 30 must be reset and the capacitors 22 must be discharged prior to removal of the demagnetized magnetic device M from or placement of a succeeding magnetic device on the holder 11.

Initiation and termination of a demagnetization operation is effected by operation of the AND circuit 31 through which the timing pulses from the pulse generator 26 must pass for operation of the digital/analog converters 25 and triggering of the SCR switching devices 24. The necessary second input signal to the AND circuit 31 is provided by a process cycle enable circuit 35. This circuit is designed to provide a second input signal for a predetermined interval of time which is adequate for completion of demagnetization operation. In the illustrated embodiment of the invention, a manual start circuit 36 connected with an input of the process cycle enable circuit may be selectively actuated to provide a signal for initiation of the operation of the latter. The process cycle enable circuit 35 is also designed to be controlled upon reception of a second input signal to terminate a demagnetization operation through elimination of the second input signal to the AND circuit 31. This second input signal for termination of a demagnetization operation is provided by an AND logic circuit 37 having five input circuits. Four of these input circuits are connected to respective input circuits to the reference shifter 30 and receive the signals from the respective level sensing switches 28. The fifth input signal for the AND circuit 37 is provided by the reference shifter 30 when the latter has received a signal from each of the level sensing switches 28 indicating conclusion of the first stage of the demagnetization operation. The signal from the reference shifter 30 to the AND circuit 37 is not applied until after the signal from tlie level sensing switches 28 has terminated and a termination signal is not transmitted to the process cycle enable circuit 35 at this time. A termination signal will only be transmitted at the conclusion of the second stage of the demagnetization operation and the AND circuit 37 has again received a signal from each of the level sensing switches 28 in addition to the signal from the reference shifter 30.

The termination signal from the AND circuit 37 is also transmitted to a reset circuit 33 operatively connected with each of respective digital/analog converters 25 and to a return reset circuit 34 connected with the reference shifter 30. Each of the reset circuits is operative to return the respective circuit to its initial state of operation. Simultaneously, the termination signal is utilized to trigger the SCR switching devices 24 to a conducting state and discharge the capacitors 22. This trigger signal is applied through the 0R circuits 32 which are also connected with the AND circuit 37. No further demagnetization will occur after discharge of the capacitors 22 since the continuous train of trigger pulses supplied by the pulse generator 26 has been interrupted by elimination of the second input signal to the AND circuit 31. At this time, the magnetic device M may be removed from the holder 11 without disruption of the level of magnetization attained through the demagnetization operation. A subsequent demagnetization operation may be initiated at this time through actuation of the manual start circuit 36.

The several electrical circuit components of this apparatus are only illustrated by representative block diagrams as the components are of a construction well known to those skilled in the art. The specific circuits of these components, being well known, may also be readily interconnected as indicated in the assembly of a demagnetizing apparatus embodying this invention. Details of construction and operating criteria for demagnetization of a particular magnetic device are within the comprehension of those skilled in the art and further specific description is omitted.

It is readily apparent from the foregoing detailed description that this invention provides a method and apparatus for effecting the demagnetization of a multipole magnetic device. The method permits simultaneous demagnetization of a multiplicity of poles on a particular magnetic device and is capable of demagnetizing each pole to a desired value with all of the poles having substantially the same resultant value. Performance of the demagnetization operation in two steps results in a relatively high degree of precision with production of relatively few magnetic devices which do not meet the desired specifications. The apparatus is automatically operable in performance of a demagnetization operation and is operable to minimize the time required for a demagnetization operation.

According to the provisions of the patent statutes, the principles of this invention have been explained and have been illustrated and described in what is now considered to represent the best embodiment. However, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

' Having thus described this invention, what is claimed 1s:

1. The method of decreasing the magnetic flux density of a multi-pole magnetic device consisting of subjecting each pole to a respective, independently controllable, relatively negative magnetic field intensity which is applied until the magnetic flux density of the specific pole is reduced to a predetermined nominal magnitude which is greater than an ultimate desired magnetic fiux density,

mined nominal magnitude continued to be applied until the magnetic flux density of each of the poles has been reduced to this magnitude.

3. The method of decreasing the magnetic flux density of a multi-pole magnetic device according to claim 1 with the magnetic field intensity being applied in sequential pulses with each pulse being of an incrementally greater magnitude.

4. The method of decreasing the magnetic flux density of a multi-pole magnetic device according to claim 3 wherein the incremental increases in magnetic field intensity are of a magnitude to effect a decrease in magnetic flux density which is slightly less than the magnitude of the difference between a maximum and a minimum ultimate desired magnetic flux density determined to be acceptable.

5. Apparatus for decreasing the magnetic flux density of a multi-pole magnetic device to an ultimate desired magnitude comprising independently operable demagnetization means adapted to be operatively associated with a respective pole of the magnetic device for subjecting the respective pole to a relatively negative magnetic field intensity, magnetic-flux-density-responsive means for providing a signal related to the magnetic flux of each of the poles, and circuit means connected with each of said demagnetization means and a respective one of said fluxdensity-responsive means to control the magnitude of the magnetic field intensity in accordance with said signal, said circuit means being operable to increase the magnetic field intensity applied to a particular pole to efiect a reduction of the magnetic flux density of the respective pole to a predetermined nominal magnitude which is greater than the ultimate desired in a first step and to further decrease'the magnetic flux density to the ultimate desired after the magnetic flux density of each of the poles has been first reduced to the nominal magnitude.

6. Apparatus according to claim 5 wherein each said demagnetization means comprises an electromagnetic inductive device adapted to be positioned in inductively coupled relationship to a respective pole of the magnetic device.

7. Apparatus according to claim 5 wherein said circuit means is operable to apply a magnetic field intensity to a specific pole in sequential pulses with each pulse being of an incrementally greater magnitude.

8. Apparatus according to claim 7 wherein said circuit means is operable, on reduction of the magnetic flux density of a specific pole to the predetermined nominal magnitude, to continue the application of pulses of magnetic field intensity to the specific pole at the last attained magnitude until the magnetic flux density of all of the poles has been reduced to the predetermined nominal value.

9. Apparatus according to claim 5 wherein said circuit means includes a selectively adjustable power source connected in circuit with said demagnetization means for the energization thereof to provide a relatively negative magnetic field intensity, a first circuit connected with said power source to control the magnitude of each pulse of magnetic field intensity applied to the magnetic device in sequentially increasing each successive pulse by a predetermined increment, and a second circuit connected with said first circuit and said magnetic-flux-densityresponsive means operable to prevent further incremental increase of successive pulses of magnetic flux intensity when the magnetic flux density of the specific pole reaches the predetermined nominal magnitude or the ultimate desired magnitude, said second circuit being connected with each other magnetic-fiux-density-responsive means and operable to permit further incremental increase of successive pulses of magnetic flux intensity applied to a specific pole when the magnetic flux density of each pole of the magnetic device is reduced to the predetermined nominal magnitude.

References Cited UNITED STATES PATENTS 2,959,722 11/1960 Gilinson 318-492 3,164,753 1/1965 Schroeder 317-123 3,300,688 1/1967 Callihan 317-123 LEE T. HIX, Primary Examiner.

J. A. SILVERMAN, Assistant Examiner. 

1. THE METHOD OF DECREASING THE MAGNETIC FLUX DENSITY OF A MULTI-POLE MAGNETIC DEVICE CONSISTING OF SUBJECTING EACH POLE TO A RESPECTIVE, INDEPENDENTLY CONTROLLABLE, RELATIVELY NEGATIVE MAGNETIC FIELD INTENSITY WHICH IS APPLIED UNTIL THE MAGNETIC FLUX DENSITY OF THE SPECIFIC POLE IS REDUCED TO A PREDETERMINED NOMINAL MAGNITUDE WHICH IS GREATER THAN AN ULTIMATE DESIRED MAGNETIC FLUX DENSITY, 